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    <div class="sect1" lang="en" xml:lang="en">
      <div class="titlepage">
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          <div>
            <h2 class="title" style="clear: both"><a id="stl_db_usage"></a>Berkeley DB configuration</h2>
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      <div class="toc">
        <dl>
          <dt>
            <span class="sect2">
              <a href="stl_db_usage.html#id1599352">Registering database and environment handles</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="stl_db_usage.html#id1599734">Truncate requirements</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="stl_db_usage.html#id1599105">Auto commit support</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="stl_db_usage.html#id1599533">Database and environment identity checks</a>
            </span>
          </dt>
          <dt>
            <span class="sect2">
              <a href="stl_db_usage.html#id1599614">Products, constructors and configurations</a>
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      <p>
        While dbstl behaves like the C++ STL APIs in most situations, there
        are some Berkeley DB configuration activities that you can and
        should perform using dbstl. These activities are described in the
        following sections.
    </p>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="id1599352"></a>Registering database and environment handles</h3>
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          </div>
        </div>
        <p>
            Remember the following things as you use Berkeley DB Database
            and Environment handles with dbstl:
        </p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
        If you share environment or database handles among multiple
        threads, remember to specify the <a href="../api_reference/C/dbopen.html#open_DB_THREAD" class="olink">DB_THREAD</a> flag in the open call to
        the handle.
    </p>
            </li>
            <li>
              <p>
        If you create or open environment and/or database handles without
        using the dbstl helper functions,
        <code class="function">dbstl::open_db()</code> or
        <code class="function">dbstl::open_env()</code>, remember that your
        environment and database handles should be:
    </p>
              <div class="orderedlist">
                <ol type="1">
                  <li>
                    <p>
                Allocated in the heap via "new" operator.
            </p>
                  </li>
                  <li>
                    <p>
                Created using the <a href="../api_reference/CXX/envcreate.html#env_DB_CXX_NO_EXCEPTIONS" class="olink">DB_CXX_NO_EXCEPTIONS</a> flag.
            </p>
                  </li>
                  <li>
                    <p>
                In each thread sharing the handles, the handles are registered using 
                either <code class="function">dbstl::register_db()</code> or
                <code class="function">dbstl::register_dbenv()</code>. 
            </p>
                  </li>
                </ol>
              </div>
            </li>
            <li>
              <p>
        If you opened the database or environment handle using the
        <code class="function">open_db()</code> or <code class="function">open_env()</code>
        functions, the thread opening the handles should not call
        <code class="function">register_db()</code> or <code class="function">register_env()</code>
        again. This is because they have already been registered by the
        <code class="function">open_db()</code> or <code class="function">open_env()</code>
        functions.  However, other threads sharing these handles still must register 
        them locally.
    </p>
            </li>
          </ul>
        </div>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="id1599734"></a>Truncate requirements</h3>
            </div>
          </div>
        </div>
        <p>
    Some Berkeley DB operations require there to be no open cursors on the
    database handle at the time the operation occurs. Dbstl is aware of
    these requirements, and will attempt to close the cursors opened in the
    current thread when it performs these operations. However, the scope of
    dbstl's activities in this regard are limited to the current thread; it
    makes no attempt to close cursors opened in other threads.  So you
    are required to ensure there are no open cursors on database handles
    shared across threads when operations are performed that require all
    cursors on that handle to be closed.
</p>
        <p>

    There are only a a few operations which require all open cursors to be closed.
    This include all container <code class="methodname">clear()</code> and
    <code class="methodname">swap()</code> functions, and all versions of
    <code class="methodname">db_vection&lt;&gt;::assign()</code> functions. These
    functions require all cursors to be closed for the database because by default
    they remove all key/data pairs from the database by truncating it.
</p>
        <p>
    When a function removes all key/data pairs from a database, there are
    two ways it can perform this activity:
</p>
        <div class="itemizedlist">
          <ul type="disc">
            <li>
              <p>
        The default method is to truncate the database, which is an
        operation that requires all cursors to be closed. As mentioned
        above, it is your responsibility to close cursors opened in other
        threads before performing this operation. Otherwise, the operation
        will fail. 
    </p>
            </li>
            <li>
              <p>
        Alternatively, you can specify that the database not be truncated.
        Instead, you can cause dbstl to delete all key/data pairs
        individually, one after another. In this situation,
        open cursors in the database will not cause the delete operations to
        fail. However, due to lock contention, the delete operations might not
        complete until all cursors are closed, which is when all their read locks 
        are released.
    </p>
            </li>
          </ul>
        </div>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="id1599105"></a>Auto commit support</h3>
            </div>
          </div>
        </div>
        <p>

    Dbstl supports auto commit for some of its container's operations. When a dbstl
    container is created using a <code class="classname">Db</code> or <code class="classname">DbEnv</code> 
    object, if that object was opened using the <a href="../api_reference/C/envset_flags.html#envset_flags_DB_AUTO_COMMIT" class="olink">DB_AUTO_COMMIT</a> flag, then
    every operation subsequently performed on that object will be
    automatically enclosed in a unique transaction (unless the operation is already in an
    external transaction). This
    is identical to how the Berkeley DB C, C++ and Java APIs behave.
</p>
        <p>
    Note that only a subset of a container's operations support auto
    commit. This is because those operations that accept or return an
    iterator have to exist in an external transactional context and so
    cannot support auto commit.
</p>
        <p>
    The dbstl API documentation identifies when a method supports auto
    commit transactions.
</p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="id1599533"></a>Database and environment identity checks</h3>
            </div>
          </div>
        </div>
        <p>
    When a container member function involves another container (for example,
    <code class="methodname">db_vector::swap(self&amp; v2)</code>), the two
    containers involved in the operation must not use the same database.
    Further, if the function is in an external or internal transaction context,
    then both containers must belong 
    to the same transactional database environment; Otherwise, the two containers
    can belong to the same database environment, or two different ones.
</p>
        <p>
    For example, if <code class="methodname">db_vector::swap(self&amp; v2)</code>
    is an auto commit method or it is in an external transaction context, 
    then <code class="literal">v2</code> must be in the same transactional database
    environment as this container, because a transaction is started
    internally that must be used by both <code class="literal">v2</code> and this
    container. If this container and the <code class="literal">v2</code> container
    have different database environments, and either of them are using transactions,
    an exception is thrown. This condition is checked in every such member
    function.
</p>
        <p>
    However, if the function is not in a transactional context,
    then the databases used by these
    containers can be in different environments because in this situation
    dbstl makes no attempt to wrap container operations in a common transaction
    context. 
</p>
      </div>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="id1599614"></a>Products, constructors and configurations</h3>
            </div>
          </div>
        </div>
        <p>
    You can use dbstl with all Berkeley DB products (DS, CDS, TDS, and HA).
    Because dbstl is a Berkeley DB interface, all necessary configurations
    for these products are performed using Berkeley DB's standard
    create/open/set APIs.
</p>
        <p>
    As a result, the dbstl container constructors differ from those of C++
    STL because in dbstl no configuration is supported using the container
    constructors.  On the other hand, dbstl container constructors accept
    already opened and configured environment and database handles. They
    also provide functions to retrieve some handle configuration, such as
    key comparison and hash functions, as required by the C++ STL
    specifications.  
</p>
        <p>
    The constructors verify that the handles passed to them are well
    configured. This means they ensure that no banned settings are used, as
    well as ensuring that all required setting are performed. If the
    handles are not well configured, an
    <code class="classname">InvalidArgumentException</code> is thrown. 
</p>
        <p>
    If a container constructor is not passed a database or environment
    handle, an internal anonymous database is created for you by dbstl. This anonymous 
    database does not provide data persistence.
</p>
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